The present invention relates to a rotor of an induction starting synchronous motor, and more particularly to the arrangement of permanent magnets provided in a core of the rotor.
In general, self-starting synchronous motors are classified into two types, that is, the wound-rotor type having field windings as secondary windings on the side of a rotor, and the permanent magnet type composed of squirrel-cage induction coils and permanent magnets. The wound-rotor type motor rotates as a wound-rotor induction motor on being started by operating a switch, and when its speed has attained a value close to the synchronous speed, the motor in turn rotates as a synchronous motor by operating the switch again to cause the motor to be synchronized with the aid of direct-current excitation induced by an exciter. The permanent magnet type motor rotates as a squirrel-cage induction motor on being started, and when its speed has attained a value close to the synchronous speed, the motor is synchronized by fluxes induced by the permanent magnets to rotate as a synchronous motor. In this motor, the switch and exciter described above are not necessary either in view of its rotor construction. It has been well-known that the permanent magnet type motor is simpler in construction and more durable in operation than the wound-rotor type.
With an induction starting synchronous motor of the permanent magnet type having a rotor composed of squirrel-cage induction coils and permanent magnets, the rotor's construction includes conclusive factors affecting smooth starting and operating characteristics, such as arrangement and characteristics of magnets and shape and size of the squirrel-cage induction coils. Although such a general tendency to be aimed has been recognized, how to design such members in practice has never been clarified.
For example, in the case of excess magnetic fluxes induced by permanent magnets provided in a rotor core, or in the case of high reluctance resulting from difference in magnetic resistance in the rotor core, it has been known that there is a tendency for starting of a motor to become difficult. In contrast herewith, in the case of insufficient magnetic fluxes induced by permanent magnets, or in the case of lower reluctance, it has also been known that there is a tendency that the pull-in operation from the state of starting and operating as an induction motor to the state operating as a synchronous motor becomes difficult. In view of these facts, it is clearly evident that a suitable arrangement and the choice of particular characteristics of the permanent magnets will greatly affect the operation of the motor for the smooth starting and the stable operation as a synchronous motor at its reliable synchronization. Therefore, suitable arrangement of the permanent magnets and selection of their material are very important.
Taking into consideration the characteristics of the permanent magnet type synchronous motor when being operated as a synchronous motor, some prior art proposals have attempted to provide an air gap in the same manner as in a permanent magnet type motor operated by an inverter in order to solve the problem of leakage fluxes occurring at ends of the permanent magnets. Particularly, in the case of the induction starting type synchronous motor, magnetic fluxes induced by the permanent magnets are often intentionally increased for facilitating an easy pull-in for synchronization, and the magnets are sometimes problematically extended in length for increasing the torque causing the motor to be synchronized. On the other hand, however, the induction coils and the permanent magnets must be arranged in a limited space. As a result, distances between the adjacent magnets forming magnetic poles necessarily become shorter with resulting increased leakage fluxes. In many cases, therefore, clearances or gaps are provided between the magnets.
As a prior art induction synchronous motor, there has been an induction motor including induction coils in the vicinity of the surface of a rotor and permanent magnets in the rotor nearer to its center than the induction coils as disclosed in, for example, Japanese Patent Application Opened Nos. 2000-301066 and 2000-287395.
FIG. 2 illustrates in a cross-section a rotor of a four-pole induction synchronous motor corresponding to the motors disclosed in these Japanese Patent Applications. In FIG. 2, there are provided in a rotor core 1 induction coils 3 and on their inner side permanent magnets a with magnetizing directions (S→N) towards side of the stator and permanent magnets b with magnetizing directions (S→N) towards side of the shaft, these magnets being located in slots 2, respectively. In order to prevent magnetic paths formed in the rotor core, that is, so-called leakage fluxes due to the closely adjacent permanent magnets a and b each other, these permanent magnets may be arranged spaced sufficiently to eliminate the influence of the leakage fluxes, or an air gap 7 may be provided between these permanent magnets. These measures can increase flux linkages with the stator so as to increase torque for synchronized rotation of the motor and to improve power factor and efficiency and further to ensure the number of fluxes required for causing the motor to be synchronized.
Not limited to four-pole motors, dimensions of permanent magnets arranged in a rotor will be structurally limited depending on the sizes and shapes of the rotor and induction coils. It is preferable to arrange the permanent magnets as near to a stator as possible to enhance magnetic coupling, leading to improved motor characteristics. This will be, however, attended by increased centrifugal forces due to the heavy magnets positioned remotely from the rotating center, thereby degrading the strength of the rotor core. Therefore, sufficient considerations must be given to arrangement of permanent magnets.
The length of the permanent magnets is limited by the induction coils 3. If the permanent magnets are located too near the stator, they may contact the induction coils and their length is obliged to be shorter. In order to elongate the permanent magnets, they can be arranged as near to the center of the rotor as possible, but with such an arrangement, the ends of the adjacent permanent magnets will be closer to each other and the permanent magnets are positioned more remotely from the stator so that leakage fluxes will increase which adversely affect the motor performance.
The width and thickness of the permanent magnets are thus limited depending upon outer diameters of the rotor and shaft hole and also the size of the induction coils. As the size of the permanent magnets has thus a limitation in design for achieving the best result, selections of suitable material and suitable dimensions and arrangement of the permanent magnets are important.
On the other hand, by arranging permanent magnets side by side in rotating directions to increase the number of magnets per one pole, the length of the magnets can be equivalently elongated. In this case, however, this consequentially increases the total amount of used magnets, resulting in increased manufacturing cost. Recently, there is an increasing demand for improving efficiency and power factor which are electrical characteristics in view of energy saving, in addition to requirements for smooth starting and operation.
As described above, the influence of the number of fluxes induced by magnets and reluctance on starting and pull-in to synchronization of a motor has been well-recognized as the tendencies, but how to design motors so as to overcome this influence has never been known. Therefore, motors sometimes experience poor starting characteristics and difficulties in starting and pull-in to the synchronization. Even if smooth starting and reliable pull-in to synchronous operation are achieved, there are cases of efficiency and power factor remaining at lower values and amount of used permanent magnets being increased with increased manufacturing cost.